Conebeam CT as an Adjunct to Digital Subtraction Myelography for Detection of CSF-Venous Fistulas

DISCUSSION

We have described a technique involving the use of CBCT as an adjunct to LDDSM for localization of CVFs in patients with spontaneous intracranial hypotension. Although LDDSM is an excellent technique for CVF localization, it has limitations that result in missed CVFs in some cases. Some of these can be found using CBCT.

The limitations of LDDSM are varied. First, although LDDSM can be performed using biplane fluoroscopy, the lateral view provides limited information in patients with a large body habitus. Therefore, we routinely use only a single anterior-posterior projection.⁴ As a result, venous opacification that overlaps meningeal diverticula or other opacified structures can be missed on the single anterior-posterior view. CBCT allows identification of these veins (Fig 1). Second, tiny flickering densities seen on DSM can be caused by pulmonary markings, motion, or subtle opacifying veins, leading to uncertainty in image interpretation. This dilemma can be obviated by performing DSM with the patient under general anesthesia, but that is challenging at some institutions and increases the procedural risk to patients. CBCT can help clarify such DSM findings and determine whether they represent true venous opacification (Fig 2). Finally, since DSM does not provide cross-sectional anatomic information, it is sometimes difficult to differentiate complex, multilobed meningeal diverticula from CVFs. CBCT provides 3D anatomic information that allows a more accurate determination (Fig 3). Additionally, characterization of the anatomic drainage of CVFs can be helpful for subsequent neurointerventional procedures, such as transvenous Onyx (Medtronic) embolization of CVFs.¹¹

Many institutions, including ours, also use lateral decubitus dynamic CTM to localize CVFs. While this technique overcomes many of the limitations of DSM, it has substantially less temporal resolution than DSM. While DSM provides continuous imaging at a high frame rate, lateral decubitus dynamic CTM allows imaging at only a few separate points in time and, as a result, may miss CVFs that opacify only intermittently after contrast injection. However, further study is needed to determine how frequently CVFs have such transient or intermittent opacification, particularly accounting for factors such as the respiratory phase during imaging and layering contrast density. Nonetheless, 1 theoretic advantage of LDDSM with CBCT is that the former provides the temporal resolution of dynamic myelography needed to identify CVFs that may only opacify intermittently, while the latter provides an option to visualize potential CVFs in 3D to clarify indeterminate DSM findings. Additionally, CBCT may have advantages over delayed decubitus CTM. We have found that CVFs seen using CBCT may not be apparent on a delayed CTM obtained as early as 15 minutes later (Figs 1–⇑3). We speculate that delayed decubitus CTM no longer has layering contrast that is sufficiently dense to visualize CVFs, even with 50-keV monoenergetic reconstructions (Figs 2–3).

Our technical report has limitations. We have only performed LDDSM with CBCT in a small number of patients and are not yet able to determine how frequently it provides additional value. To date, we have used CBCT in 15 patients who had indeterminate findings on initial review of their LDDSM. In 7 cases, a CVF was found using CBCT. Five of these patients have undergone transvenous Onyx embolization for treatment, and 4 have had clinical follow-up documenting resolution of symptoms (1 patient has not yet been seen for follow-up). As more data are accrued, further study will be needed to better elucidate the yield of CBCT. Additionally, other modifications to our typical LDDSM technique, such as the use of biplane fluoroscopy, may sometimes be a better alternative to CBCT. Comparison between CBCT and lateral decubitus dynamic CTM will also be helpful. Nonetheless, we have encountered cases in which CBCT is an invaluable adjunctive tool to identify CVFs and currently consider it a worthwhile technique.